This invention relates to molten chloride solvent extractive distillation to separate hafnium from zirconium and in particular relates to a zinc chloride-lead chloride solvent to provide the liquid phase.
Naturally occurring zirconium ores generally contain from 1 to 3 percent hafnium oxide relative to zirconium oxide. In order that the zirconium metal be acceptable as a nuclear reactor material, the hafnium content must first be reduced to low levels, due to the high neutron absorption cross section of hafnium. This separations process is difficult due to the extreme chemical similar of the two elements. A number of techniques have been explored to accomplish this separation, with the technique currently in use in the United States involving liquid-liquid extraction of aqueous zirconyl chloride thiocyanate complex solution using methyl isobutyl ketone, generally as described in U.S. Pat. No. 2,938,769, issued to Overholser on May 31, 1960, with the removal of iron impurities prior to solvent extraction as described in U.S. Pat. No. 3,006,719, issued to Miller on Oct. 31, 1961.
Several processes have been suggested for separations of the metal tetrachlorides generated from the ore by carbochlorination. The use of such a nonaqueous separation offers significant economic incentive over those processes requiring aqueous zirconium solutions. Direct distillation of the tetrachlorides provides one possible route, relying on the difference in boiling points between zirconium tetrachloride and hafnium tetrachloride. Unfortunately, direct distillation cannot be accomplished at near atmospheric pressure, since neither tetrachloride exhibits a liquid phase except at very high pressure. U.S. Pat. No. 2,852,446, issued to Bromberg on Sept. 16, 1958 describes a high pressure distillation process where the pressure, rather than a solvent, provides for a liquid phase.
U.S. Pat. No. 1,582,860, issued to van Arkel and de Boer on Apr. 27, 1926 provides a molten salt extractive distillation process utilizing what they described as a zirconium (hafnium) tetrachloride-phosphorus oxychloride compound. U.S. Pat. No. 2,816,814 issued to Plucknett on Dec. 17, 1957, describes extractive distillation for separation of the tetrachlorides using a stannous chloride solvent. U.S. Pat. No. 2,928,722 to Scheller, issued Mar. 15, 1960, describes the batch fractional distillation of niobium and tantalum chlorides to separate these chlorides from each other and from other chloride impurities, and uses a "flux" to provide the molten salt phase, utilizing either zirconium tetrachloride-phosphorus oxychloride or an alkali metal chloride and aluminum (or iron, or zirconium) chloride mixture as the flux. U.S. Pat. No. 3,966,458 issued to Spink on June 29, 1976 provides a sodium-potassium chloride solvent for use in the extractive distillation of zirconium and hafnium tetrachlorides. U.S. Pat. No. 3,671,186 issued to Ishizuka on June 20, 1972, utilizes a series of dissolution and evaporation stages with a solvent such as sodium chloride. U.S. Pat. No. 4,021,531 issued to Besson on Apr. 3, 1977, utilizes extractive distillation with an alkali metal chloride and aluminum (or iron) chloride mixture as the solvent. Extractive distillation of zirconium (hafnium) tetrachloride with a pure zinc chloride solvent has been attempted (Plucknett et al., AEC report ISC-51, 1949), but was unsuccessful due to the formation of a highly viscous two-phase system. The anomalously high viscosity of zinc chloride is described by MacKenzie and Murphy (J. Chem. Phys., 33, 366, 1960).
Of all of the molten salt extractive distillation processes, only the above mentioned Besson process has been brought to commercial development. This process is currently in use in France and provides product zirconium tetrachloride, relatively depleted of hafnium tetrachloride in the liquid bottoms stream and a hafnium tetrachloride enriched vapor stream taken from the top of the column. A relatively high reflux is provided by a condenser at the top of the column and a reboiler at the bottom of the column. Because of the stability of the double salts formed with the alkali metal chloride in the solvent, it is very difficult to completely separate the product zirconium tetrachloride from the solvent, and relatively high (e.g. 500.degree. C.) temperatures are required. Aluminum chloride in excess of 1:1 molar to alkali metal chloride is required and there is considerable carry over of aluminum chloride into the zirconium tetrachloride leaving the stripper. French Pat. No. 2,543,162 (9/28/84) to Brun and Guerin describes a post-stripping process for removing aluminum chloride. In addition, it should be noted that aluminum chloride is an especially corrosive molten salt, and, at higher temperatures, is very difficult to handle.